Headphones can include a loudspeaker for only one ear, or for both ears. The loudspeakers can sit outside the ear or in the ear. Some headphones include additional features such as built-in microphones so that they can be used as headsets.
Users desire to control the level of audio played by the headphones. When the user shuts the audio source off and then restarts it, or connects the headphones to a different source, the initial audio level may be incorrect for the user. This can cause the user to have to change the audio level each time the headphones are used. In extreme circumstances the initial audio level could be so high as to have the potential of physically harming the user.